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So there might be one abc for several payments, now I have:
//find abc id for each payment id
Map<Long, Integer> abcIdToPmtId = paymentController.findPaymentsByIds(pmtIds)
.stream()
.collect(Collectors.toMap(Payment::getAbcId, Payment::getPaymentId));
But then I reallize this could have duplicate keys, so I want it to return a
Map<Long, List<Integer>> abcIdToPmtIds
which an entry will contain one abc and his several payments.
I know I might can use groupingBy but then I think I can only get Map<Long, List<Payments>> .
753
HashMap stores key, value pairs and it does not allow duplicate keys. If the key is duplicate then the old key is replaced with the new value.
You can use a TreeMap with a custom Comparator in order to treat each key as unequal to the others. It would also preserve the insertion order in your map, just like a LinkedHashMap. So, the net result would be like a LinkedHashMap which allows duplicate keys!
Get the stream of elements in which the duplicates are to be found. For each element in the stream, count the frequency of each element, using Collections. frequency() method. Then for each element in the collection list, if the frequency of any element is more than one, then this element is a duplicate element.
A TreeMap cannot contain duplicate keys. TreeMap cannot contain the null key. However, It can have null values.
Use the other groupingBy overload.
paymentController.findPaymentsByIds(pmtIds)
.stream()
.collect(
groupingBy(Payment::getAbcId, mapping(Payment::getPaymentId, toList());
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